The present disclosure relates generally to turbine engines and, more specifically, to systems and methods of mitigating the formation of thermal rotor bow in turbofan engines.
Known turbofan engines experience several different phases of operation including, but not limited to, startup, warmup, steady-state, shutdown, and cool-down. Turbofan engines may cycle through the different phases of operation several times a day depending on the use of the aircraft in which the turbofan engines are attached. For example, a commercial passenger aircraft typically shuts down its engines in between flights for safety purposes as passengers disembark from the aircraft. As such, residual heat remains in the aircraft's engines, which can cause a phenomenon known as thermal rotor bow. Thermal rotor bow is generally defined by deformation in the rotating components of the turbofan engine, such as the rotating drive shafts. Deformation in the rotating components of the turbofan engine can result in performance deterioration due to rubs and/or contact-related damage between the rotating and stationary components of the turbofan engine during engine startup, thereby reducing the service life of the turbofan engine.
Thermal rotor bow is especially prominent at times after engine shutdown, and before the engine is allowed to fully cool. Moreover, many known turbofan engines are unable to naturally mitigate thermal rotor bow during startup as the design of modern commercial turbofans shifts towards having higher bypass ratios and greater length-to-diameter ratios. More specifically, increasing the length-to-diameter ratio of the turbofan engines facilitates reducing the resonant frequency of the rotating assembly to potentially below engine idle speed. In the presence of thermal rotor bow, this will produce a vibratory response during engine startup. Such a vibratory response can cause unwanted aircraft-level effects in addition to physical damage to engine components. One known method of mitigating thermal rotor bow is to motor the turbofan engine with a starter motor to lessen the severity of the rotor bow prior to fuel introduction and subsequent progression to idle. However, motoring the turbofan engine with the starter motor during startup can be a time-consuming and inconvenient process. Another known method of mitigating thermal rotor bow is to introduce a cooling fluid into one of the compressor bleed ports to displace heated air within the engine. However, cooling fluid introduced into the compressor bleed port often is not channeled to a proper location in the gas turbine engine to be able to facilitate mitigating thermal rotor bow.